M. K. A. Zittel on Fossil HexactinelUda. 257 



to the right, and is formed at the expense of the hexagonal 

 subanal plate and the posterior and right genital plates, 

 which are the smallest. The disk is pentagonal, and the 

 madreporic plate is slightly the largest. Next in size is the 

 plate to the left. The ocular plates are well developed, the 

 three anterior being the largest. The anus has a membrane, 

 and under it yjlates supporting small spines. All the surface 

 of the apical system is crowded with short, blunt, cylin- 

 drical, round-tipped, reticulate, and lobose processes, between 

 which are pedicellarite ; the processes are found on the anal 

 margin, on the anal membrane, and they project from the 

 free surface and edges of tlie plates. 



EXPLANATION OF PLATE VII. 



Fic). \. Salenia profundi, Dune, X|-. 



Ficj. 2. Anal openinor, map-nified. 



Fif;/. 3. An ocular plate and part of a generative plate, magnified. 



Fiff. 4. Club-shaped spines and a pedicellaria, magnified. 



Fiff. o. A short serrate spine, magnified. 



Fif/. 6. A spine at the peristome end of an ambulacrum, magnified. 



Fif/. 7. A tentacle of a pore, magnified. 



Fif/. 8. Diagram of the apical disk. 



Fir/. 9. A pair of buccal tentacles, pedicellariae, and membrane, magnified. 



Fif/. 10. A gill. 



Fi(/. 11. A large spine, slightly magnified. 



XXXIV. — Studies on Fossil Sponges. — I. HexactinelUda. 

 By Karl Alfred Zittel*. 



Systematic Position of the HexactinelUda. 



Among the numerous discoveries of O. Schmidt in the de- 

 partment of spongology, none has acquired such widely influ- 

 ential importance, from a palajontological point of view, as the 

 establishment and definition of the orders Hexactinellida and 

 Lithistidaf. Wyville Thomson J had indeed previously indi- 

 cated the relationship of the Ventriculites of the English Chalk 

 to certain living siliceous sponges ; but it was only after 

 Schmidt had shown that the so-called " glass-sponges " {Vi- 



* Translated by W. S. Dallas, F.L.S., from a separate copy of the 

 memoir in the ' Abhandlungen der k. bayer. Akad. der Wiss.' II. CH. 

 Band xiii. 1877, communicated by the author. 



t ' Grundziige einer Spongienfauna des Atlantischen Gebietes,' Leipzig, 

 1876. 



X 'The Depths of the Sea,' Royal Dublin Society, April 1869, and 1873, 

 p. 483. 



